Conventionally, an RFID system where a reader/writer identifies a tag by transmitting a radio wave of approximately one watt from the reader/writer, by receiving the signal on the tag side, and by returning information within the tag to the reader/writer with a radio wave, has been put into practical use.
For this RFID system, a radio signal of a frequency of the UHF (Ultra High Frequency) band (865 MHz in EU, 915 MHz in US, and 953 MHz in JP) is used.
In a tag, an LSI (Large Scale Integrated) chip and an antenna are directly connected in normal cases. The pattern of the antenna is formed by etching Cu evaporated onto an insulative sheet such as a film, paper, etc. or by coating with an Ag paste. Normally, the size of the antenna pattern is approximately 100 to 150 mm×10 to 25 mm.
If the antenna of the tag is a normal dipole antenna, a communication distance between the reader/writer and the tag is approximately 3 to 5 m, although it depends on the operating power of the LSI chip of the tag.
Additionally, as an antenna that can extend the communication distance between the reader/writer and the tag, a circular loop antenna that is small enough to fit within an area of 97.5 mm2 by 54 mm2 is proposed (for example, see “Size Reduction in UHF Band RFID Tag Antenna Based on Circular Loop Antenna”, Hong-Kyun Ryu; Jong-Myung Woo; Applied Electromagnetics and Communications, 2005. ICECom 2005. 18th International Conference on 12-14 Oct. 2005 Page(s): 1-4).
Since the RFID tag is normally used by being attached to a commodity, etc., it is generally designed in consideration of the permittivity, the thickness, etc., of an object to which the tag is attached.
However, if such a normal tag described above is attached to a metal, a radio wave emitted from the reader/writer is not picked up by the tag, or an antenna gain becomes extremely small because the metal to which the tag is attached serves as an obstacle. Therefore, the emission of a radio wave returned from the tag cannot be obtained.
This is also similar in the above described dipole antenna and circular loop antenna.
To solve this problem, an antenna of a completely different shape becomes necessary. For example, a loop antenna that uses metal surfaces has been used, on the contrary, for a long time.
FIG. 1 is an explanatory view of the principle of a conventional loop antenna that uses metal surfaces. This figure schematically illustrates a state where a tag 4 composed of an LSI chip 2 and a loop antenna 3 is made to contact a surface of a metal 1 (viewed from the side of the metal 1, the metal 1 being in the form of a plate).
The loop antenna 3 is composed of a top 5, a bottom 6 and both sides 7 of a loop. The loop antenna 3 is arranged so that the bottom 6 of the loop is positioned along a surface of the metal 1 and the loop is made orthogonal to the surface of the metal 1.
Here, when a radio wave from the reader/writer is emitted in a direction indicated by an arrow 8, an electric current in a direction indicated by arrows 9 is induced in the loop antenna 3 of the tag 4.
The loop of the loop antenna 4 is arranged orthogonal to the surface of the metal 1 as described above. Therefore, the electric current induced in the loop antenna 4 forms the eddy current indicated by the arrows 9 on the surface orthogonal to the surface of the metal 1.
If an eddy current occurs on a surface orthogonal to one of a surfaces of a metal, the metal surface normally works as if it was a mirror, and an electric current component that flows in a mirror image path 5′, 6′ and 7′, indicated by a broken line in a direction indicated by arrows 9′ (direction reverse to the previously mentioned eddy current) in FIG. 1, also occurs orthogonally to the other surface of the metal and symmetrically to the original surface. This phenomenon is called a mirror image effect.
If mutually opposing eddy currents occur at positions that are orthogonal to and symmetrical with the metal surface as described above, the electric current components at the bottom 6 and in the mirror image path 6′ of the loop in the metal surface portion on both of the surfaces of the metal cancel each other out, and only electric current components at the top 5 and both of the sides 7 of the loop, and in the mirror image path 5′ and 7′ remain.
The remaining current components form an eddy current component that flows along both of the surfaces of the metal as if it penetrated through the metal surface, as virtually illustrated with a solid line 10. As a result, the loop antenna 3 can obtain a very large antenna gain.
FIG. 2 illustrates an equivalent circuit of the LSI chip 2 and the loop antenna 3 of the above described tag 4. The LSI chip 2 normally includes a parallel resistance Rc (approximately 200 to 2000Ω) and a parallel capacitance Cc (approximately 0.2 to 2 pF).
FIG. 3 is an equation for calculating a condition under which the above described LSI chip and loop antenna match at a predetermined resonance frequency. f0, L and C represent the resonance frequency, an inductance and a capacitance, respectively.
Here, to make the LSI chip 2 and the loop antenna 3 of the tag 4 illustrated in FIG. 1 match, it is known to be preferable that the parallel inductance La of the loop antenna 3 and the parallel capacitance Cc of the LSI chip 2 cancel each other out if the parallel resistance Ra of the loop antenna 3 illustrated in FIG. 2 has the same value as the parallel resistance Rc of the LSI chip 2 and if the parallel inductance La of the loop antenna 3 exists in the relationship of FIG. 3.
At this time, all of the induced power of the radio wave received by the loop antenna 3 is supplied to the LSI chip 2. Moreover, all of the power from the LSI chip 2 is supplied to the loop antenna 3, and is externally emitted.
In the meantime, the loop antenna has a nature such that its loop length is automatically determined when the size and the permittivity ∈r of a substrate holding the loop antenna are determined.
Accordingly, if the loop antenna 3 has a parallel inductance component La that satisfies the equation in FIG. 3 in the tag 4 that takes the shape illustrated in FIG. 1 and includes the equivalent circuit illustrated in FIG. 2, the loop antenna 3 matches the LSI chip 2. However, sometimes the value of the parallel inductance component La does not reach a value that satisfies the equation of FIG. 3, depending on the size or the permittivity ∈r of the holding substrate.
FIG. 4 illustrates a simulation model created to conduct a performance test of the loop antenna 3 of the tag 4 schematically illustrated in FIG. 1.
In the model tag 11 illustrated in FIG. 4, the size of the cuboid, namely, the size of the longer side×the shorter side×the thickness is set to 50.8 mm×25.4 mm×5.4 mm. Originally, an LSI chip is connected to a feeding part at the ends of both of the feeding terminals 130 at the center of the loop antenna 120. However, a simulation port surface 140 is formed here.
It should be assumed that this loop antenna 120 is formed by pasting copper (Cu) foil onto the surfaces of the holding substrate 150 that is insulative and slightly transparent. It should also be assumed that the entirety of the surfaces of the tag 11 are molded by a resin for environmental resistance, although the mold resin is not illustrated due to its transparency.
Additionally, an LSI chip to be mounted on the port surface 140 is actually the size of an LSI package that protects and accommodates the LSI chip. Therefore, the size of the LSI package is assumed to be 10 mm×10 mm.
Furthermore, it should be assumed that the permittivity ∈r of the holding substrate 150 and the mold resin is 3.7. In this configuration, it should also be assumed that the parallel resistance Rc of the LSI chip, which is made to match the loop antenna 120, is 1000 to 2000Ω, and the parallel capacitance Cc is 0.8 pF in the equivalent circuit illustrated in FIG. 2.
To make the loop antenna 120 match this LSI chip, it is most ideal, based on the equation of FIG. 3, that the parallel resistance Ra of the loop antenna 120 be 1000 to 2000Ω, and the parallel inductance La be 35 nH.
According to calculation results obtained by simulating the above described model under the above described conditions with a commonly sold electro-magnetic field simulator, Ra and La are respectively 8000Ω and 20 nH, which are far from the above described ideal values, and do not match the LSI chip at all.
The capacitance Cc of the LSI chip that can cope with the loop antenna having Ra of 8000Ω and La of 20 nH, which are obtained from the simulation, is 2.0 pF on the basis of the equation represented by FIG. 3. Such an LSI chip for a tag is impractical.
Here, assuming that the permittivity ∈r of the holding substrate 150 is increased to approximately 10, the parallel inductance La of the loop antenna 120 is in the vicinity of 35 nH. Therefore, this loop antenna matches the LSI chip.
However, ceramics having a very high permittivity ∈r are forced to be used as the holding substrate 150 in this case. A normal holding substrate 150 is currently commonly sold at a price of approximately 100 yen, while a ceramic substrate taking the same shape costs more than 1000 yen. Accordingly, the cost of the entire tag increases, which is not cost-effective.
Additionally, if the size of the holding substrate 150 is increased to approximately 80×50 mm, the loop length of the loop antenna formed on the surface of the holding substrate 150 also becomes longer with an increase in the size of the holding substrate 150. Then, the parallel inductance component La of the loop antenna ends up in the vicinity of 35 nH, which almost matches the LSI chip having a parallel resistance Rc of 1000 to 2000Ω and a parallel capacitance Cc of 0.8 pF.
In this case, however, the loop antenna, namely, the holding substrate, becomes huge, and exceeds a practical size as a tag.